This invention relates to a rotary drum device in a video tape recorder in which a magnetic head is made movable in the widthwise direction of the magnetic tape so that the magnetic record tracks are traced with high accuracy.
FIG. 4 is a sectional view showing essential components of a conventional rotary drum device. FIG. 5 is a diagram showing essential components of the rotary drum device as viewed in the direction of the arrows substantially along line V--V in FIG. 4. FIG. 6 is a diagram showing a magnetic head drive section as viewed in the direction of the arrows substantially along line VI--VI in FIG. 4. FIGS. 7 and 8 are a sectional view and a side view of the magnetic head drive section shown in FIG. 6, respectively.
In FIGS. 4 and 5, reference numeral 1 designates a lower drum; 2, a bearing coupled to the lower drum 1; 3, a rotary shaft turned while being supported by the bearing 2; 4, a base stand fitted on one end portion of the rotary shaft 3; 5, an upper drum secured to the base stand 4 with screws 6; 7, the aforementioned magnetic head drive section secured to the upper drum 5 with screws 8; 9, a lower transformer mounted on the lower drum 1; 10, an upper transformer secured to the base stand 4; 11, a wiring board mounted on the upper drum 5; 12, a stationary contact unit for supplying control current to the magnetic head drive section; 13, a rotary electrode unit installed on a part of the base stand 4 so as to contact the contact unit 12; 14, a connecting part for electrically connecting the electrode unit 13 through a connecting part 15 and the wiring board 11 to the magnetic head drive section 7; and 16, a magnetic head secured to the magnetic head drive section 7. The magnetic head 16 is electrically connected through a connecting part 17, the wiring board 11 and the connecting part 15 to the upper transformer 10.
Further in FIGS. 4 and 5, reference numeral 18 designates a recess formed in a part of the upper drum 5 to receive the magnetic head drive section 7, the recess 18 being made larger than the magnetic head drive section 7 to permit the adjustment in position of the magnetic head 16; 19, position adjusting holes for adjusting the position of the magnetic head; and 20, a magnetic tape held in slide contact with the magnetic head 16.
FIGS. 6, 7 and 8 show the magnetic head drive section 7 in more detail. In these figures, reference numeral 21 designates a first yoke of magnetic material; 22, a first permanent magnet secured to the first yoke, the first permanent magnet 22 being in the form of a bar; 23, a second yoke of magnetic material, the second yoke 23 being secured to the first yoke 21 and having an annular protrusion 23b extended from a part of the inner cylindrical wall thereof; 24, a third yoke of magnetic material secured to the second yoke; 25, a second permanent magnet in the form of a bar which is secured to the third yoke 24 in such a manner that the same magnetic poles of the first and second permanent magnets confront with each other; 26, a pole piece disposed between the first and second permanent magnets 22 and 25 and secured to one of the first and second permanent magnets; and 27, a leaf spring of non-magnetic material held between the first and second yokes 21 and 23. The leaf spring 27 has a prolongation 27a which extends through a window (21a and 23a) formed in the first and second yoke 21 and 23, and has a magnetic head 16 at the end.
Further in FIGS. 6, 7 and 8, reference numeral 28 designates a leak spring of non-magnetic material held between the second yoke and the third yoke; 29, fixing members held by the leaf springs 27 and 28, respectively; 30, a bobbin bonded to the fixing members 29 with adhesive 32 with a predetermined gap F around the outer cylindrical walls of the first permanent magnet 22, the second permanent magnet 25 and the pole piece 26; and 31, a coil formed by winding a covered wire on the bobbin in such a manner that a predetermined gap G is formed between the coil 31 and the annular protrusion 23b.
The function of the conventional rotary drum device thus constructed will be described with reference to FIG. 7. The first permanent magnet 22 sets up a magnetic flux D in a closed magnetic path of the pole piece 26, the second yoke 23 and the first yoke 21. Similarly, the second permanent magnet 25 sets up a magnetic flux E in a closed magnetic path of the pole piece 26, the second yoke 23 and the third yoke 24. The magnetic flux E is opposite in direction to the magnetic flux D. Thus, the magnetic fluxes D and E go across the annular gaps F and G in the same direction, and the total of the magnetic fluxes set up by the first and second permanent magnets 22 and 25 goes across the coil 31. When, under this condition, current is applied through the contact unit 12, the electrode unit 13 and the connecting parts 15 and 14, the coil 31, the bobbin 30 and the magnetic head 16 are moved, as one unit, linearly in a vertical direction.
Thus, the magnetic head 16 is displaced in the widthwise direction of the magnetic tape 16, to trace the magnetic record tracks.
In the conventional rotary drum device described above, the material of the bobbin 30 is different from that of the fixing members 29. Therefore, in order to secure the fixing members to the bobbin, it is necessary to use a two-solution mixing type adhesive which is expensive. And it is rather troublesome to secure the fixing members to the bobbin with the adhesive. In addition, the adhesion between the fixing members and the bobbin is low in reliability, and accordingly the rotary drum device itself is low in reliability.